Add at rest encryption to Meterpreter payloads

[sjanusz-r7]

This PR adds in the generation of encrypted payload files using ChaCha20.
The files are automatically encrypted when the Rake tasks are called.
Using #read allows for the usage of encrypted and plain-text payloads as before.

ChaCha20 was chosen based on the following benchmarks on my developer machine:

First run:

~/Programming/benchmarks via 🐍 v3.11.5 via 💎 v3.0.5 
❯ ruby ./src.rb    
Warming up --------------------------------------
   plaintext binread     1.468k i/100ms
aes encrypted read (decrypting)
                       877.000  i/100ms
chacha20 encrypted read (decrypting)
                       785.000  i/100ms
rc4 encrypted read (decrypting)
                       459.000  i/100ms
Calculating -------------------------------------
   plaintext binread     14.506k (±10.1%) i/s -     71.932k in   5.009154s
aes encrypted read (decrypting)
                          8.835k (±12.0%) i/s -     43.850k in   5.034825s
chacha20 encrypted read (decrypting)
                          6.886k (±14.0%) i/s -     33.755k in   5.004575s
rc4 encrypted read (decrypting)
                          3.876k (±11.4%) i/s -     19.278k in   5.041633s

Comparison:
   plaintext binread:    14506.2 i/s
aes encrypted read (decrypting):     8835.0 i/s - 1.64x  slower
chacha20 encrypted read (decrypting):     6885.6 i/s - 2.11x  slower
rc4 encrypted read (decrypting):     3876.0 i/s - 3.74x  slower

Second run:

~/Programming/benchmarks via 🐍 v3.11.5 via 💎 v3.0.5 took 28s 
❯ ruby ./src.rb
Warming up --------------------------------------
   plaintext binread     1.214k i/100ms
aes encrypted read (decrypting)
                       881.000  i/100ms
chacha20 encrypted read (decrypting)
                       801.000  i/100ms
rc4 encrypted read (decrypting)
                       395.000  i/100ms
Calculating -------------------------------------
   plaintext binread     12.861k (± 7.5%) i/s -     64.342k in   5.032598s
aes encrypted read (decrypting)
                          7.570k (±15.3%) i/s -     37.002k in   5.003907s
chacha20 encrypted read (decrypting)
                          7.901k (±12.6%) i/s -     39.249k in   5.051235s
rc4 encrypted read (decrypting)
                          3.951k (±11.1%) i/s -     19.750k in   5.063716s

Comparison:
   plaintext binread:    12861.5 i/s
chacha20 encrypted read (decrypting):     7900.5 i/s - 1.63x  slower
aes encrypted read (decrypting):     7569.5 i/s - 1.70x  slower
rc4 encrypted read (decrypting):     3951.0 i/s - 3.26x  slower